Staged venting of fuel cell system during rapid shutdown

ABSTRACT

A venting methodology and system for rapid shutdown of a fuel cell apparatus of the type used in a vehicle propulsion system. H 2  and air flows to the fuel cell stack are slowly bypassed to the combustor upon receipt of a rapid shutdown command. The bypass occurs over a period of time (for example one to five seconds) using conveniently-sized bypass valves. Upon receipt of the rapid shutdown command, the anode inlet of the fuel cell stack is instantaneously vented to a remote vent to remove all H 2  from the stack. Airflow to the cathode inlet of the fuel cell stack gradually diminishes over the bypass period, and when the airflow bypass is complete the cathode inlet is also instantaneously vented to a remote vent to eliminate pressure differentials across the stack.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 09/502,640 filed Feb. 11, 2000, now U.S. Pat. No. 6,395,414.

GOVERNMENT SUPPORT

The Government of the United States of America has right in thisinvention pursuant to Agreement No. DE-AC02-90CH10435 awarded by theU.S. Department of Energy.

FIELD OF THE INVENTION

This invention relates to a fuel cell system and more particularly to asystem having a plurality of cells which consume an H₂-rich gas toproduce power.

BACKGROUND OF THE INVENTION

Fuel cells have been used as a power source in many applications. Forexample, fuel cells have been proposed for use in electrical vehicularpower plants to replace internal combustion engines. In proton exchangemembrane (PEM) type fuel cells, hydrogen is supplied to the anode of thefuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuelcells include a membrane electrode assembly (MEA) comprising a thin,proton transmissive, non-electrically conductive solid polymerelectrolyte membrane having the anode catalyst on one of its faces andthe cathode catalyst on the opposite face. The MEA is sandwiched betweena pair of electrically conductive elements which (1) serve as currentcollectors for the anode and cathode, and (2) contain appropriatechannels and/or openings therein for distributing the fuel cell'sgaseous reactants over the surfaces of the respective anode and cathodecatalysts. The term fuel cell is typically used to refer to either asingle cell or a plurality of cells (stack) depending on the context. Aplurality of individual cells are commonly bundled together to form afuel cell stack and are commonly arranged in series. Each cell withinthe stack comprises the membrane electrode assembly (MEA) describedearlier, and each such MEA provides its increment of voltage. A group ofadjacent cells within the stack is referred to as a cluster. Typicalarrangements of multiple cells in a stack are described in U.S. Pat. No.5,763,113, assigned to General Motors Corporation.

In PEM fuel cells, hydrogen (H₂) is the anode reactant (i.e., fuel) andoxygen is the cathode reactant (i.e., oxidant). The oxygen can be eithera pure form (O₂), or air (a mixture of O₂ and N₂). The solid polymerelectrolytes are typically made from ion exchange resins such asperfluoronated sulfonic acid. The anode/cathode typically comprisesfinely divided catalytic particles, which are often supported on carbonparticles, and mixed with a proton conductive resin. The catalyticparticles are typically costly precious metal particles. These membraneelectrode assemblies are relatively expensive to manufacture and requirecertain conditions, including proper water management andhumidification, and control of catalyst fouling constituents such ascarbon monoxide (CO), for effective operation.

For vehicular applications, it is desirable to use a liquid fuel such asan alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline)as the source of hydrogen for the fuel cell. Such liquid fuels for thevehicle are easy to store onboard and there is a nationwideinfrastructure for supplying liquid fuels. However, such fuels must bedissociated to release the hydrogen content thereof for fueling the fuelcell. The dissociation reaction is accomplished within a chemical fuelprocessor or reformer. The fuel processor contains one or more reactorswherein the fuel reacts with steam and sometimes air, to yield areformate gas comprising primarily hydrogen and carbon dioxide. Forexample, in the steam methanol reformation process, methanol and water(as steam) are ideally reacted to generate hydrogen and carbon dioxide.In reality, carbon monoxide and water are also produced. In a gasolinereformation process, steam, air and gasoline are reacted in a fuelprocessor which contains two sections. One is primarily a partialoxidation reactor (POX) and the other is primarily a steam reformer(SR). The fuel processor produces hydrogen, carbon dioxide, carbonmonoxide and water. Downstream reactors may include a water/gas shift(WGS) and preferential oxidizer (PROX) reactors. In the PROX carbondioxide (CO₂) is produced from carbon monoxide (CO) using oxygen fromair as an oxidant. Here, control of air feed is important to selectivelyoxidize CO to CO₂.

Fuel cell systems which process a hydrocarbon fuel to produce ahydrogen-rich reformate for consumption by PEM fuel cells are known andare described in co-pending U.S. patent application Ser. Nos. 08/975,422and 08/980,087, filed in November, 1997 now U.S. Pat. Nos. 6,332,005 and6,077,620, respectively, and U.S. Ser. No. 09/187,125, filed inNovember, 1998 now U.S. Pat. No. 6,238,815 and each assigned to GeneralMotors Corporation, assignee of the present invention; and inInternational Application Publication Number WO 98/08771, published Mar.5, 1998. A typical PEM fuel cell and its membrane electrode assembly(MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issuedrespectively Dec. 21, 1993 and May 31, 1994, and assigned to GeneralMotors Corporation.

Efficient operation of a fuel cell system depends on the ability toeffectively control gas flows (H₂ reformate and air/oxygen) to the fuelcell stack not only during start-up and normal system operation, butalso during system shutdown. During the shutdown of a fuel cell systemthat generates hydrogen from liquid fuel, the anode CO emissionsincrease and can degrade the stack. Accordingly, a primary concernduring shutdown is diverting the gas flows of H₂ and air/oxygen aroundor away from the fuel cell stack and disposing of the excess H₂. The H₂and air flows being diverted from the stack during shutdown must also bekept separate to avoid creating a combustible mixture in the system. Thestack must also be protected from prolonged (e.g., greater than fiveseconds) pressure differentials which could rupture the thin membranesin the membrane electrode assembly (MEA) separating the anode andcathode gases.

Fuel cell systems, in particular those used in vehicular applications,are often used to generate start-up and transient heat for the fuelprocessor. The combustor is fueled by the anode and cathode effluents,supplemental hydrocarbon fuel for start-up and high demand situations,and excess H₂ from the fuel processor. The combustor is also useful forburning off residual stack effluents and processor H₂ during systemshut-down. During normal system operation, the combustor typically runsat a constant temperature, for example around 600° Celsius in anexemplary vehicle propulsion system application. It is important at alltimes to prevent the combustor from overheating, as the resultingdegradation would require an expensive replacement and would interferewith the operation of the system as a whole. The combustor thereforegenerally receives a continuous air flow from the system air supply. Airflow to the combustor must be maintained during shutdown to preventoverheating as the combustor burns off residual gases.

The cooling of the combustor therefore competes with the shutdownobjectives of gas flow diversion and residual H₂ combustion. Especiallywhere the air supply to the system generally supplies both the combustorand the cathode inlet of the fuel cell stack, the diversion and ventingof air from the cathode inlet must not even temporarily deprive thecombustor of sufficient airflow for cooldown.

During normal shutdown of the system in which time is not a factor, thecompeting demands of gas flow diversion and combustor cooldown arerelatively easy to offset and satisfy. However, during rapid shutdown,carbon monoxide emissions at the stack anode and pressure differentialsat the cathode need to be dissipated in a few seconds. At the same time,sufficient air flow must be maintained to the combustor for thelengthier cooldown period. The coordinated diversion and venting of thegas flows with respect to both the fuel cell stack and combustor becomesdifficult.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a venting methodology for stagingthe diversion and venting of reformate H₂ and air relative to the fuelcell stack, the combustor, and one or more vents. This staged ventingprotects the stack from degradation due to CO and due to high pressuredifferentials, and protects the combustor from overheating. In anotheraspect, the invention further provides a currently-preferred valving andcontrol scheme for carrying out the venting methodology.

In a fuel cell system in which the fuel cell stack and the combustor aresupplied with H₂ and air, respectively, by a common H₂ supply and acommon air supply, and each of the H₂ supply and air supply is providedwith a bypass valve which supplies both the stack and the combustorduring normal system operation but which bypasses the stack to thecombustor during shutdown, the stack anode inlet is instantaneouslyvented as the bypass valves are commanded to close. The air flow ratiois slowly shifted between the cathode inlet and the combustor by the airsupply bypass valve until the air flows almost entirely to thecombustor. The cathode inlet is vented at a point during the air flowratio shift at which venting will not significantly affect the flow ofcooling air to the combustor, but before the pressure differentialbetween the cathode and anode inlets can degrade the membranes in thestack.

According to another feature of the invention methodology, the H₂ supplypath to the combustor is vented simultaneously with the stack anodeinlet.

According to another feature of the invention methodology, the H₂ fromthe anode inlet and the air from the cathode inlet are vented throughseparate vents to prevent the creation of a combustible mixture in thesystem. Both of the H₂ and air vents preferably vent to atmospherealthough other arrangements (adsorbers, holding tanks) might be usefulfor certain applications.

According to another aspect of the invention, the invention methodologyis carried out by fast-acting vent valves provided in the flow path ofthe H₂ bypass valve to the anode inlet; in the H₂ supply path from theH₂ bypass valve to the combustor; and in the air supply path between theair bypass valve and the cathode inlet. The vent valving for carryingout the invention methodology may comprise existing valves and a fuelcell system controlled according to the invention methodology during arapid shutdown, or may comprise single-purpose valving added to anexisting fuel cell system. Control of the vent valving can be through adedicated controller comprising any suitable microprocessor,microcontroller, personal computer, etc. which has a central processingunit capable of executing a control program and data stored in thememory. The controller may additionally comprise an existing controllerin a fuel cell system.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features, advantages and other uses of the present inventionwill become more apparent by referring to the following description anddrawings in which:

FIG. 1 is a drawing depicting a fuel cell system to which the stagedventing methodology and a preferred venting arrangement according to thepresent invention can be applied.

FIG. 2 is a drawing of the fuel cell system shown in FIG. 1 connected ina pictorial representation of a use application.

FIG. 2A is a flow diagram depicting an exemplary generation of normaland rapid shutdown commands by an onboard vehicle system.

FIG. 3 is a simplified, idealized gas flow and venting representation ofthe fuel cell system of FIG. 1 provided with a venting arrangementaccording to the present invention, in a normal operating (i.e.,non-shutdown) mode.

FIG. 4 illustrates the fuel cell system of FIG. 3 in a first stage of arapid shutdown mode in which the bypass valves are just beginning toclose.

FIG. 5 illustrates the fuel cell system of FIG. 3 in an intermediatestage of rapid shutdown, in which the bypass valves are partiallyclosed.

FIG. 6 illustrates the fuel cell system of FIG. 3, in which the bypassvalves are near-closed or fully closed.

FIG. 6A illustrates an alternate embodiment of the system of FIGS. 3-6,which assumes a pressure-resistant fuel cell stack.

FIG. 7 is a flow diagram representation of the venting methodologyillustrated by the valve positions in FIGS. 3-6.

FIG. 8 illustrates the fuel cell system of FIG. 1 with vent valvingadded to carry out the staged venting methodology of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention methodology and apparatus for venting a fuel cellsystem during rapid shutdown provides a staged venting and diversion ofgas flows to permit adequate combustor cooldown without damaging thefuel cell stack.

The invention is particularly useful for fuel cell systems used toproduce power for vehicle propulsion. This may be further understoodwith reference to the fuel cell system shown in FIG. 1 by way of exampleonly. Therefore, before further describing the invention, it is usefulto understand the type of system within which the staged ventingmethodology can be employed to protect the stack and combustor, andfurther to illustrate the location and interplay of existing ventvalving in such a system.

FIG. 1 illustrates an example of a fuel cell system. The system may beused in a vehicle (not shown) as an energy source for vehiclepropulsion. In the system, a hydrocarbon is processed in a fuelprocessor, for example, by reformation and preferential oxidationprocesses, to produce a reformate gas which has a relatively highhydrogen content on a volume or molar basis. Therefore, reference ismade to “H₂” as hydrogen-rich or having a relatively high hydrogencontent.

The invention is hereafter described in the context of a fuel cellfueled by an H₂-rich reformate regardless of the method by which suchreformate is made. It is to be understood that the principles embodiedherein are applicable to fuel cells fueled by H₂ obtained from anysource, including reformable hydrocarbon and hydrogen-containing fuelssuch as methanol, ethanol, gasoline, alkene, or other aliphatic oraromatic hydrocarbons.

As shown in FIG. 1, a fuel cell apparatus includes a fuel processor 2for catalytically reacting a reformable hydrocarbon fuel stream 6, andwater in the form of steam from a water stream 8. In some fuelprocessors, air is also used in a combination preferentialoxidation/steam reforming reaction. In this case, fuel processor 2 alsoreceives an air stream 9. The fuel processor contains one or morereactors 12 wherein the reformable hydrocarbon fuel in stream 6undergoes dissociation in the presence of water/steam 8 and sometimesair (in stream 9) to produce the hydrogen-rich reformate. Further, eachreactor 12 may comprise one or more reactor beds. Reactor 12 may haveone or more sections or beds, and a variety of designs are known andusable. Therefore, the selection and arrangement of reactors 12 mayvary; and exemplary fuel reformation reactor(s) 14 and downstreamreactor(s) 16 are described immediately below.

By way of example, in an exemplary steam/methanol reformation process,methanol and water (as steam) are ideally reacted in a reactor 14 togenerate hydrogen and carbon dioxide as described earlier in thebackground. In reality, carbon monoxide and water are also produced. Byway of further example, in an exemplary gasoline reformation process,steam, air and gasoline are reacted in a fuel processor which comprisesa reactor 14 which has two sections. One section of the reactor 14 isprimarily a partial oxidation reactor (POX) and the other section of thereactor is primarily a steam reformer (SR). As in the case of methanolreformation, gasoline reformation produces the desired hydrogen but, inaddition, produces carbon dioxide, water and carbon monoxide. Therefore,after each type of reformation, it is desirable to reduce the carbonmonoxide content of the product stream.

Accordingly, the fuel processor typically also includes one or moredownstream reactors 16, such as water/gas shift (WGS) and preferentialoxidizer (PROX) reactors which are used to produce carbon dioxide fromcarbon monoxide, as described earlier in the background. Preferably, theinitial reformate output gas stream which comprises hydrogen, carbondioxide, carbon monoxide and water is further treated in a preferentialoxidation (PROX) reactor 16 to reduce the CO-levels therein toacceptable levels, for example, below 20 ppm. Then, during running mode,H₂ rich reformate 20 is fed through valve 31 into the anode chamber of afuel cell stack 22. At the same time, oxygen (e.g., air) from an oxidantstream 24 is fed into the cathode chamber of the fuel cell 22. Thehydrogen from the reformate stream 20 and the oxygen from the oxidantstream 24 react in the fuel cell 22 to produce electricity.

Exhaust or effluent 26 from the anode side of the fuel cell 22 containssome unreacted hydrogen. The exhaust or effluent 28 from the cathodeside of the fuel cell 22 contains some unreacted oxygen. Air for theoxidant stream 24 is provided by an air supply, preferably compressor30. Air from the air supply (compressor 30) is directed to the fuel cell22 by a valve 32 under normal operating conditions. During start-up,however, the valve 32 is actuated to provide air directly to the inputof a combustor 34. The air is used in combustor 34 to react with a fuelsupplied through line 46. The heat of combustion is used to heat variousparts of the fuel processor 2.

It should be noted that some of the reactions which occur in fuelprocessor 2 are endothermic and so require heat; other reactions areexothermic and require removal of heat. Typically, the PROX reactor 16requires removal of heat. One or more of the reformation reactions inreactor 14 are typically endothermic and require heat to be added. Thisis typically accomplished by preheating the reactants (fuel 6, steam 8,and air 9) and/or by heating selected reactors.

Heat from the combustor 34 heats selected reactors and reactor beds inthe fuel processor 2 during start-up. The combustor 34 achieves heatingof the selected reactors and beds in the fuel processor, as necessary,by indirect heat transfer thereto. Typically, such indirectly heatedreactors comprise a reaction chamber with an inlet and an outlet. Withinthe reaction chamber, the beds are in the form of carrier membersubstrates each having a first surface carrying catalytically activematerial for accomplishing the desired chemical reactions. A secondsurface opposite the first surface is for heat transfer from hot gasesto the carrier member substrates. In addition, the combustor 34 isusable to preheat the fuel 6, water 8 and air 9 being supplied asreactants to the fuel processor 2.

It should be noted that the air 9 supplied to the fuel processor 2 maybe used in one or more of the reactors 12. If reactor 14 is a gasolinereformation reactor, then air from line 9 is supplied to reactor 14. ThePROX reactor 16 also utilizes air to oxidize CO to CO₂ and also receivesair from air supply source (compressor 30) via line 9.

The combustor 34 defines a chamber 41 with an inlet end 42, an exhaustend 44 and a catalyst section 48 between the ends. Hydrocarbon fuel isinjected into the combustor. The hydrocarbon fuel, if in liquid form, ispreferably vaporized either before being injected into the combustor orin a section of the combustor to disperse the fuel for combustion.Vaporization may be done by an electric heater. Once the system isoperating and the combustor has heated up, vaporization may occur byheat exchange using heat from the combustor exhaust to vaporize incomingfuel. Preferably, a fuel metering device 43 is provided to control therate at which hydrocarbon fuel is provided to the combustor.

The hydrocarbon fuel 46 and the anode effluent 26 are reacted in thecatalyst section 48 of the combustor 34, which section is between theinlet and exhaust ends 42 and 44, respectively, of the combustor 34.Oxygen is provided to the combustor 34 either from the air supply (i.e.,compressor 30) via valve 32 or from a second air flow stream, such as acathode effluent stream 28, depending on system operating conditions. Avalve 50 permits release of the combustor exhaust 36 to atmosphere whenit is not needed to heat reactors in the fuel processor 2.

As can be seen, the hydrocarbon fuel stream 46 supplements the anodeeffluent 26 as fuel for the combustor 34, as may be needed, to meet thetransient and steady state needs of the fuel cell apparatus. In somesituations, exhaust gas passes through a regulator 38, a shutoff valve140 and a muffler 142 before being released to the atmosphere. In FIG.1, the symbols are as follows: V is valve, MFM is mass flow meter, T istemperature monitor, R is regulator, C is cathode side, A is anode sideof fuel cell, INJ is injector, and COMP is compressor.

The amount of heat demanded by the selected reactors within the fuelprocessor 2, which is to be supplied by the combustor 34, is dependentupon the amount of fuel and water input and ultimately the desiredreaction temperature in the fuel processor 2. As stated earlier,sometimes air is also used in the fuel processor reactor and must alsobe considered along with the fuel and water input. To supply the heatdemand of the fuel processor 2, the combustor 34 utilizes all anodeexhaust or effluent and potentially some hydrocarbon fuel. Enthalpyequations are used to determine the amount of cathode exhaust air to besupplied to the combustor 34 to meet the desired temperaturerequirements of the combustor 34 and the combustor 34 ultimatelysatisfies the heat demanded by the fuel processor 2. The oxygen or airprovided to the combustor 34 includes one or both of cathode effluentexhaust 28, which is typically a percentage of the total oxygen suppliedto the cathode of the fuel cell 22, and a compressor output air streamdepending on whether the apparatus is operating in a start-up modewherein the compressor air stream is exclusively employed, or in a runmode using the cathode effluent 28 and/or compressor air. In the runmode, any total air, oxygen or diluent demand required by the combustor34, which is not met by the cathode effluent 28, is supplied by thecompressor 30 in an amount to satisfy the temperature and heat demandedby the combustor 34 and the fuel processor 2, respectively. The aircontrol is implemented via an air dilution valve 47 which preferably isa stepper motor driven valve having a variable orifice to control theamount of bleed-off of cathode exhaust 28 supplied to the combustor 34.

In this exemplary representation of a fuel cell apparatus, operation isas follows. At the beginning of operations when the fuel cell apparatusis cold and starting up: (1) the compressor 30 is driven by an electricmotor energized from an external source (e.g., a battery) to provide thenecessary system air; (2) air is introduced into the combustor 34;hydrocarbon fuel 46 (e.g., MeOH or gasoline) is injected into thecombustor 34; (3) the air and fuel react in the combustor 34, wheresubstantially complete combustion of the fuel is effected; and (4) thehot exhaust gases exiting the combustor 34 are conveyed to the selectedreactors 12 associated with the fuel processor 2.

Once the reactors in the fuel processor 2 have attained adequatetemperature, the reformation process begins and the process includes thefollowing: (1) valve 32 is activated to direct air to the cathode sideof the fuel cell 22; (2) fuel and water are fed to the fuel processor 2to commence the reformation reaction; (3) reformate exiting the fuelprocessor 2 is fed to the anode side of the fuel cell 22; (4) anodeeffluent 26 from the fuel cell 22 is directed into the combustor 34; (5)cathode effluent 28 from the fuel cell 22 is directed into the combustor34; (6) the fuel, air, cathode effluent 28 and anode effluent 26 areburned in the combustor 34. In a preferred sequence, step (2) isimplemented first along with the supplying of air directly to thecombustor. Then, when the hydrogen-rich stream has adequately low COlevel, steps (1) and (3) are implemented, followed by steps (4), (5) and(6).

Under certain conditions, the combustor 34 could operate solely on theanode and cathode effluents, without the need for additional hydrocarbonfuel 46. Under such conditions, fuel injection to the combustor 34 isdiscontinued. Under other conditions, e.g., increasing power demands,supplemental fuel 46 is provided to supplement the Aout (26) to thecombustor 34. It can be seen that the combustor 34 receives multiplefuels, such as a hydrocarbon fuel as well as anode effluent 26 from theanode of the fuel cell 22. Oxygen depleted exhaust air 28 from thecathode of the fuel cell 22 and air from the compressor 30 are alsosupplied to the combustor 34.

According to the present fuel cell system example, a controller 150shown in FIG. 1 controls various aspects of the operation of the systemshown in FIG. 1. The controller 150 may comprise any suitablemicroprocessor, microcontroller, personal computer, etc., which has acentral processing unit capable of executing a control program and datastored in a memory. The controller 150 may be a dedicated controllerspecific to any of the components in FIG. 1, or implemented in softwarestored in the main vehicle electronic control module. Further, althoughsoftware based control programs are usable for controlling systemcomponents in various modes of operation as described above, it willalso be understood that the control can also be implemented in part orwhole by dedicated electronic circuitry.

In a preferred embodiment, the fuel cell system uses the fuel cell 22 aspart of a vehicle propulsion system (see FIG. 2). Here, a portion of thepropulsion system 60 comprises a battery 62, an electric motor 64, andassociated drive electronics in the form of an inverter 65, constructedand arranged to accept electric energy from a DC/DC converter 61associated with the fuel cell system, and particularly fuel cell 22, andto convert it to mechanical energy produced by motor 64. The battery 62is constructed and arranged to accept and store electrical energysupplied by fuel cell 22 and to accept and store electrical energysupplied by motor 64 during regenerative braking, and to provideelectric energy to motor 64. The motor 64 is coupled to driving axle 66to rotate wheels of a vehicle (not shown). An electrochemical enginecontrol module (EECM) 70 and a battery pack module (BPM) 71 monitorvarious operating parameters, including, but not limited to, the voltageand current of the stack. For example, this is done by the battery packmodule (BPM) 71, or by the BPM 71 and the EECM 70 together, to send anoutput signal (message) to the vehicle controller 74 based on conditionsmonitored by the BPM 71. The vehicle controller 74 controls the electricmotor 64, the inverter 65, the DC/DC converter 61, and requests a powerlevel from the EECM 70.

The gas flows (H₂ and air) to the fuel cell 22 and combustor 34 in thefuel cell system of FIG. 1 have been described for a start-up mode and arun mode. Such systems also have a shutdown mode in which the gas flowsto the fuel cell 22 are diverted and finally terminated, for examplewhen a vehicle using the fuel cell system for propulsion is turned off.This diversion and termination of gas flow is accomplished throughpreviously-illustrated valves 31 and 32 for the H₂ and air flows,respectively. In the illustrated system for vehicle propulsion, valves31 and 32 typically take the form of automotive type bypass valves,usually solenoid-operated ball valves with a pipe diameter of around 1to 1½ inches. These are generally three-way valves (one input, twopossible outputs) whose function includes bypassing the flow of H₂ andair from fuel cell 22 to combustor 34 during shutdown.

Air flow to the combustor through valve 32 prevents the combustor fromoverheating as it burns off residual H₂ bypassed from valve 31 andeffluent tapered off from the anode outlet of fuel cell 22. Continuedair flow then promotes cooldown of the combustor after all residual H₂has been burned off. A typical operating temperature for a combustorused in a fuel cell apparatus of the type illustrated in FIG. 1 is 600°C. Overheating can degrade the combustor, requiring expensive repairs orreplacement. Accordingly, providing sufficient air flow to the combustorduring shutdown, both to maintain a constant temperature for residualburn off and then for combustor cooldown, must be given a priorityduring the shutdown procedure.

The controller 150 shown in FIG. 1, which may be implemented by way ofnon-limiting example in the BPM 71 and/or the EECM 70, monitors theoperation of the fuel cell system with respect to pressures,temperatures, start-up times, cycles, etc. and routinely generatesshutdown commands in response to selected transition conditions of thesystem for transmittal to algorithm logic (see FIG. 2A).

The system shutdown control according to the present invention may beimplemented in either hardware or software. Preferably, the control isimplemented in software as part of the control program on the controller150. FIG. 2A is an exemplary representation of control as a logiccircuit, as disclosed in U.S. patent application Ser. No. 09/345,139 nowU.S. Pat. No. 6,159,626, [H-204426] [GMFC-4426] co-owned with thepresent application by the assignee of the present application. Thelogic in FIG. 2A examines each shutdown command signal received fromcontroller 150 and makes a determination or differentiation with respectto whether the shutdown command should be viewed as a rapid shutdowncommand or a normal shutdown command. The differentiation involvesexamining criteria which are briefly illustrated in FIG. 2A, and whichare described in detail in the co-pending application referred to above.The details of the rapid shutdown command decision and signal generationare not critical to the present invention, whose staged ventingmethodology and vent valving arrangements are capable of use with manydifferent forms of rapid shutdown command schemes.

Rapid shutdown is significantly shorter in duration than normalshutdown. In the event of a rapid shutdown, standard automotive bypassvalving 31, 32 of FIG. 1 cannot provide the desired response timewithout expensive modification. Automotive type bypass valves 31 and 32are relatively small and move slowly to the bypass position (forexample, one to five seconds). One way to provide for a rapid shutoffwould be to increase the size and speed of valves 31 and 32. If valves31 and 32 are electric valves, faster valve operation would require alarger solenoid actuator. If valves 31 and 32 are pneumatic valves,faster valve operation would require larger diaphragm actuators. In bothcases, however, the substitution of larger, more expensive valves issimply not practical for high volume automotive applications. And largervalves may require more electric power or air pressure than is readilyavailable on a given vehicle.

In addition to consideration of the speed at which the bypass valves actin a rapid shutdown situation, care must always be taken to avoiddegradation the fuel cell stack, since carbon monoxide concentrationtends to increase during shutdown.

Also, the relatively delicate membranes in the stack cannot toleratesignificant pressure differentials between the cathode and anode gasesfor prolonged periods, for example more than five seconds. It would bepreferable to avoid any significant pressure differential across thestack during the typical one to five second operating period of bypassvalving such as 31 and 32.

FIGS. 3-6 illustrate a preferred embodiment of the invention methodologyand preferred venting apparatus for use with a system such as thatillustrated in FIG. 1. It will be understood that FIGS. 3-6 aresimplified illustrations based on the system shown in FIG. 1. FIG. 7 isa flow diagram representation of the venting methodology illustrated bythe valve positions in FIGS. 3-6. FIG. 8 illustrates the fuel cellsystem of FIG. 1 with vent valving added to carry out the staged ventingmethodology of the present invention. FIG. 6A illustrates an alternateembodiment of the system of FIGS. 3-6, which assumes apressure-resistant fuel cell stack. The invention allows for rapidshutdown of the fuel cell system using standard, slow-acting automotivebypass valves 31 and 32, while providing adequate cooling air to thecombustor during shutdown and protecting the fuel cell 22 from carbonmonoxide degradation and prolonged pressure differentials.

In FIG. 3, a fuel cell system according to the invention is shown priorto a rapid shutdown. The fuel cell system illustration of FIG. 3 is asimplified version of that shown in FIG. 1, emphasizing gas flows, valveoperation, and additional vent valving for carrying out the invention.The additional vent valving is shown as vent valve 80 in the line orpath 20 supplying H₂ from supply 2 through bypass valve 31 to anodeinlet 22 a; vent valve 82 in the line or path supplying H₂ from supply 2through bypass valve 31 to combustor 34; a combustible vent 84 whichreceives vented H₂ from vent valves 80 and 82; a vent valve 86 in theline or path 24 supplying air from supply 30 through bypass valve 32 tothe cathode inlet 22 b; and, oxidant vent 88 for receiving air ventedfrom vent valve 86. Optional check valving 90, 92 can be providedbetween the anode outlet 22 c and the cathode outlet 22 d and thecombustor to prevent backflow in the flow paths.

In a preferred form vent valves 80, 82 and 86 are fast-acting solenoidvents. Combustible vent 84 and oxidant vent 88 may simply discharge toatmosphere, and are kept separate to avoid creating a combustiblemixture of H₂ and air in the system during the venting process.

It will be understood that although vent valves 80, 82 and 86 arefast-acting, nearly instantaneous-opening valves, their simple one-waynature allows them to be smaller than the more complicated, multi-pathbypass valves 31 and 32. The new vent valving accordingly does not placea significant burden in terms of power consumption or size on the fuelcell system, or on any related vehicle system.

It will also be understood that although vents 84 and 88 are preferablysimple discharges to atmosphere, they may take other forms such as, butnot limited to, holding tanks, adsorber beds, and other known devicesfor storing or handling gas flows.

FIG. 4 represents the fuel cell system according to the invention justafter controller 150 has determined the need for a rapid shutdown andsent appropriate control signals to the fuel cell system. Both valves 31and 32 are commanded to “bypass” fuel cell 22, and begin closing overtheir predetermined time period. Just prior to or simultaneouslytherewith, the anode vent solenoids 80 and 82 are commanded to open, andthey do so in a manner which can be considered instantaneous as comparedto the closing time of bypass valves 31 and 32. The open position isillustrated in FIG. 4 by the open circles representing the location ofvent valves 80 and 82.

In FIG. 5, bypass valves 31 and 32 have partially closed, and anodevents 80 and 82 have already vented hydrogen completely from the anodeinlet 22 a. Cathode inlet vent 86 remains closed. Air continues to flowto combustor 34 through fuel cell 22 via cathode inlet 22 b, cathodeoutlet 22 d, and path 28. At this point the anode side of fuel cell 22is near barometric pressure, but the cathode side of the fuel cell is atthe relatively high pressure of the air supply. Since this condition haslasted less than five seconds, the pressure differential has not beenprolonged enough to degrade the membranes in the fuel cell stack.

In FIG. 6, bypass valves 31 and 32 are completely closed, i.e. they havecompletely diverted H₂ and air from fuel cell stack 22 and now are openonly to combustor 34 through lines 20 a and 24 a, respectively. Shortlybefore or simultaneous with the closing of cathode bypass valve 32 hasclosed, cathode inlet vent valve 86 is commanded to open andinstantaneously vents the accumulated air pressure on the cathode sideof the fuel cell stack to oxidant vent 88. This eliminates the pressuredifferential across the fuel cell stack, protecting the membranes fromrupture.

The methodology of staged venting just described in reference to FIGS.3-6 is illustrated in step-by-step flowchart form in FIG. 7.

During the rapid shutdown, combustor 34 receives sufficient airflow tofirst prevent overheating as it burns off residual H₂, and then to cooldown once all residual H₂ is combusted. In general vent valve 86 mustremain closed until the cathode bypass valve 32 is nearly or fullyclosed, otherwise it would sap or draw cooling air away from combustor34.

The opening of cathode inlet vent 86 not only rids the stack of pressuredifferential, but in case a membrane in the stack ruptures, it vents H₂or “breakthrough” methanol from the stack away from the combustor.

Depending on combustor operation during normal run mode of the fuel cellapparatus, the flow of cooling air to the combustor during shutdown mayactually increase with the foregoing invention. For example, thecathode/combustor airflow ratio during run mode may be 100/0 or 80/20.As cathode bypass valve 32 begins to close during a rapid shutdown, thisratio will gradually shift: 80/20; 50/50; 20/80; until, finally, itreaches 0/100, at which point the stack is bypassed by the air supplyand the combustor is receiving all of the air produced by the air supplyfor residual combustion and cooling.

The foregoing invention takes advantage of the ability of the fuel cellstack membranes to tolerate a short period of relatively high pressuredifferential at the cathode in order to ensure sufficient cooling air tocombustor 34 during a rapid shutdown. However, as fuel cell developmentadvances, and as systems capable of better withstanding pressure oroperating under lower pressures are introduced, it may be possible toutilize the invention without cathode bypass valve 32 and bypass pathway24. Referring now to FIG. 6A, a fuel cell system similar to that shownin FIGS. 3-6 is illustrated, and assumes a fuel cell stack capable ofhandling cathode air flow without anode gas flow. The system of FIG. 6Aalso assumes a membrane capable of withstanding a prolonged pressuredifferential, and resistant to the membrane drying which occurs withcontinued air flow when there is no electrical demand on the stack.Assuming such a fuel cell, the staged venting methodology of theinvention is still important to the fuel cell apparatus in order to 1)maintain sufficient cooling air to combustor 34; and 2) vent the cathodeside of the fuel cell if stack membrane degradation occurs, therebyventing hydrogen leaked to the cathode side and preventing air frombeing leaked to the anode side. In such an arrangement the (H₂) cathodevent valve 86 remains closed until all of the fuel is vented or burnedfrom the system, and then the cathode side of the stack is vented tovent 88. After venting, cathode vent valve 86 is closed for the longduration cooldown of the combustor and fuel processor.

It will be understood from the foregoing examples of the inventionmethodology and apparatus that a particular method and valvingarrangement is illustrated for one exemplary fuel cell system. Thespecific valving arrangement, location of valves, types of valves andvents used, the relative speeds of the valving and their closingfunction relative to one another may vary depending on the fuel cellapparatus to which the invention is applied. Such variations andmodifications can be made by those skilled in the art without undueexperimentation now that we have disclosed our invention in theembodiment above. Nothing in the foregoing description is intended tolimit the invention beyond the scope of the following claims.

What is claimed is:
 1. In a fuel cell system comprising a fuel cellstack having anode and cathode inlets and outlets, a combustor, an H₂supply in gas flow communication with the anode inlet and the combustor,and an air supply in gas flow communication with the cathode inlet andcombustor, wherein the combustor relies primarily on the air supply forcooling airflow, a venting system for rapidly shutting down the fuelcell system comprising: an anode bypass valve selectively providing gasflow communication between the H₂ supply and the anode inlet andcombustor, the anode bypass valve having a relatively slow speed ofoperation in which it bypasses the anode inlet over a predetermined timeperiod; a first instantaneous anode vent; a second instantaneous H₂supply vent; a third instantaneous cathode inlet vent; and, a controllerfor activating the first and second instantaneous vents and the anodebypass valve in response to a rapid shutdown of the fuel cell system,and for activating the third instantaneous vent at approximately the endof the predetermined time period.
 2. The system of claim 1, furtherincluding a cathode bypass valve selectively providing gas flowcommunication between the air supply and the cathode inlet andcombustor, and further wherein the controller activates the cathodebypass valve in response to the rapid shutdown command.